The central branch of the Denver Public Library system boasts one of the most beautiful buildings in the city. But it continues to fight a reputation for attracting some skeezy patrons -- and that battle won't be made any easier by a new revelation.

Turns out that the library's security personnel haven't always reported to police about sexually suspect acts that take place on the premises. And while some of these incidents don't qualify as actual crimes, quite a few others certainly do.

The info comes to us courtesy of 9News, which received a tip from a woman who was at the library when she noticed a man looking at her and busily masturbating.

Inside the library.

At that point, she picked up a courtesy phone and informed security what was going on, and officers responded -- although the man was able to skip out before he could be grabbed.

Problem is, she subsequently discovered that security didn't call the cops -- an oversight she considers "reprehensible." (An investigation is now underway, and surveillance footage may give detectives a look at the man doing...well, you know.)

This wasn't the only slip-up of its kind. The station discovered via an open records request that there have been sixty -- count 'em, sixty -- incidents of a sexual nature internally recorded at the library thus far in 2013. The majority of those involved people looking at porn on facility computers, which isn't illegal but is against the rules for obvious reasons: Who wants to catch a glimpse of goggle-eyed dudes watching the horizontal mambo on the way to check out a reference book? The computers are set up to prevent porn access, but apparently some folks are pretty good at getting around the blockade.

Bob Knowles.

Other matters are more serious, though, including indecent exposure, sex in the bathrooms and even pissing on books -- and central branch security manager Bob Knowles admitted to the station that not every one of these offenses has triggered a call to the cops.

One such oversight: A guy was caught peeping child porn on a computer, but a security guard allowed him to leave.

That guard has since been informed of proper policy, and Knowles emphasizes that others on the staff have received a similar message. Good thing, too -- if the library wants to eliminate any hint of skeeziness.